Efforts to build on a "very fragile" Syrian ceasefire are accelerating, with detailed plans to deploy UN observers within days being drawn up to keep up pressure on President Bashar al-Assad for a peaceful outcome to the bloodiest chapter of the Arab spring.

Kofi Annan, the international envoy, has told the UN security council that a truce was being partially observed, though the regime and the opposition accused each other of breaching it in the first hours after it began at 6am.

Annan, representing the UN and the Arab League, said the Syrian government had not withdrawn its troops, tanks and heavy weapons from cities as it was supposed to do 48 hours before Thursday's truce came into effect.

"Syria is experiencing a rare moment of calm on the ground," he said in a statement.

"The cessation of hostilities appears to be holding … This is bringing much-needed relief and hope to the Syrian people who have suffered so much for so long in this brutal conflict."

Ban Ki-Moon, the UN secretary-general, called the ceasefire "very fragile".

Syrian state TV blamed "terrorists" for a bomb attack in Aleppo, while diplomats and opposition sources reported incidents of soldiers swapping their uniforms for civilian clothes to mislead any outside observers. Expectations are high, based on previous experience, that the government will try to manipulate the situation.

"It's clear that the regime hasn't implemented what it promised, but there are straws we can clutch at," said one senior western diplomat.

The US secretary of state, Hillary Clinton, speaking after a meeting of G8 foreign ministers, warned that Assad must implement the peace plan in full. "The Annan plan is not a menu of options. It is a set of obligations," she said. "For it to be meaningful, this apparent halt in violence must lead to a credible political process and a peaceful, inclusive democratic transition." There were fears among the G8 that the ceasefire might not hold until the observers could arrive.

Nato also said it was monitoring the situation , after the Turkish foreign minister, Ahmet Davutoglu, said Turkey might seek help from the alliance if there were any further incursions into its territory. Syrian forces opened fire across the Turkish border on Monday, killing two people in a Turkish refugee camp.

In Syria on Thursday opposition activists reported violence by security forces in Zabadani, Hama and Homs. The network of Local Coordination Committees counted 15 people killed by shelling and sniper fire in Homs, Qusair, Bayada, Idlib and the Damascus region. Troops also reportedly fired on demonstrators outside the parliament in Damascus.

Anti-Assad forces could face a significant test on Friday if plans for mass protests go ahead. Burhan Ghalioun, head of the opposition Syrian National Council, said: "It will be the biggest possible [demonstration] so that the Syrian people can express their will." The right to peaceful assembly is one plank of Annan's six-point plan. It also calls for humanitarian and Arab and foreign media access.

Ricken Patel of the campaigning group Avaaz said: "Having used the Annan plan as cover, killed 1,000 people and crushed the remaining pockets of the Free Syrian Army, of course Assad is happy to now observe a ceasefire. The question is, will he keep it when the people of Syria flood into the streets on Friday?"

Worryingly, the Syrian interior ministry said in a statement that only pre-authorised demonstrations would be permitted by police. It also called on opposition fighters "whose hands have not been stained with Syrian blood" to hand themselves in with their weapons with a promise they would be freed without charge.

News agencies on Thursday night obtained a draft resolution being considered by the UN security council that would allow an initial 30 observers in to monitor the ceasefire. Diplomatic sources had earlier said this could rapidly become a 500-strong mission. The draft demanded that the government ensure freedom of movement for the observers and the ability to interview anyone they want in private.

The UN will be keen to avoid a repeat of the situation last December when the authorities were accused of misleading the Arab League monitoring mission.

"It will be no good if they are able to deploy in Homs and Hama if the regime can do what it wants unobserved in Idlib and Deraa," one official said.

The UN draft also demanded that the government withdraw troops and heavy weapons from population centres. If Syria does not implement its commitments, the draft expresses the council's determination "to consider further measures as appropriate".

Annan would also have to make clear what would constitute a sufficiently serious breach of the ceasefire to render it void.

The UN official looking at the arrangements, Major-General Robert Mood of Norway, underlined the difficulties. "Both sides are plagued by a very high degree of mutual suspicion," he said. "It's terribly difficult for them to cross that abyss."

Syria's ambassador to the UN, Bashar Ja'afari, said his government was "on board" with the peace plan. But he said that government forces would remain on alert to "counterattacks" and that "legally speaking, there is a big difference between declaring a ceasefire and putting an end to the violence".